In any chromogenic photographic material it is desirable that the dyes formed should be bright in colour, absorbing light in the appropriate spectral region, with very little secondary absorption so that good colour reproducibility is obtained. It is also important that the formed photographic images should be resistant to fading due to heat, humidity and light. Many of the desired hue characteristics are achieved by changing the molecular structure of the photographic dye-forming coupler, hereinafter coupler. For example alkyl substituents on a pyrazolotriazole nucleus will form magenta dyes on photographic development, whereas carbonamido-substituted pyrazolotriazoles form cyan dyes. In addition, adjusting the environment of the coupler can influence the dye hue of the azomethine dye that is formed during photographic processing. For example, the spectral properties of the dyes can be altered using hue-shifting solvents such as sulfonamides (EP-A-0 309 159, EP-A-0 264 083), carbonamides (e.g. U.S. Pat. Nos. 4,935,321 and 5,258,278), aryl ureas (U.S. Pat. No. 4,808,502), ballasted and substituted phenols (U.S. Pat. No. 4,562,146, DE 3,936,300), sulfones and sulfoxides (U.S. Pat. No. 5,232,821) and phosphate esters (EP-A-0 515 128).
For economic and environmental reasons, a reduction in the quantities of materials used in photographic materials is desirable. Couplers derived from bicyclic or even tricyclic heterocyclic compounds are increasingly being utilised because they can form dyes with higher absorptivities than those that have been used for many years (such as phenolic and naphtholic cyan couplers). Thus, on an equimolar basis, these heterocyclic couplers can provide much higher dye densities from the same amounts of silver, so that it is possible to generate the same dye densities from lower laydowns of silver and coupler.
Whilst pyrazolo-based couplers have been used in photographic products for magenta dyes, their use in the formation of cyan dyes is more problematical. Several patents exist which refer to this class of cyan couplers and most of them address the two problems associated with them: they form dyes which are highly aggregated, that is they form a self-associating assembly of monomeric molecules, and the light stability of these dyes is very poor.
Dye aggregation is a major concern for these couplers because, although the desired spectral absorptions of these dyes are due to the monomeric species, the unwanted secondary absorptions are caused by the presence of aggregated dye. These secondary absorptions have a deleterious effect on the quality of the image that is produced so that the hues of a scene are rendered much less accurately than required. Hue-shifting by solvents alone is of little use and a de-aggregating compound is also required.
U.S. Pat. No. 5,294,528 lists a variety of agents that can be used to break the aggregation of an azomethine dye (more correctly an azamethine dye), in particular the dye from a magenta class of pyrazolotriazole couplers. It is claimed in this patent that “azomethine dyes formed from pyrazoloazole couplers are liable to aggregate, and the higher the aggregation degree of the dyes is, the lower the light-fastness is, and that by breaking the aggregation the light-fastness of azomethine dyes can be enhanced”. Whilst it is true that some magenta pyrazoloazole couplers with bulky substituents provide dyes which are less aggregated and have better light stability than those with less bulky substituents, de-aggregation is not the main cause of improved light stability. A de-aggregating compound is understood to act by intervention within the dispersed oil phase containing the aggregated dye, and is necessarily a component of the coupler dispersion. Compounds which de-aggregate in this way may perform quite different functions if included in separate dispersions, e.g. as UV absorbers. Furthermore, compounds which act as stabilizers may benefit in a synergistic, rather than a simple additive, way when included in the same dispersion as the coupler and de-aggregating agent. We have found that de-aggregating compounds do not necessarily provide extra light stability unless another appropriate light stabilizer is present. Thus improved light stability is not correlated with de-aggregation.
EP-A-0 886 179 suggests that liquid crystalline solvents can be used to improve colour reproducibility for a variety of heterocyclic coupler classes, including pyrazolo-based couplers. EP-A-0 883 024 and U.S. Pat. No. 6,132,945 indicate that cyclic imide materials and phenyl carboxylic acid derivatives improve the colour reproduction of azomethine dyes produced by pyrrolotriazole couplers. U.S. Pat. No. 6,007,975 suggests that a phenolic coupler can be combined with various heterocyclic cyan couplers to improve colour reproduction.
All of these have been investigated and have problems with them, not least that, whilst dye aggregation can be reduced, light stability has not been improved sufficiently to become practicable. The reason for this is that pyrazolo-based couplers which form cyan dyes on photographic development differ from magenta ones in that they have electron-withdrawing functionalities attached to the heterocyclic nucleus. This appears to make them less amenable to dye stabilization via the methods used for their magenta counterparts. These methods to improve dye stability involve using compounds such as, for example, spiroindanes (U.S. Pat. No. 5,212,055), bisphenols (U.S. Pat. No. 3,700,455), thiomorpholine dioxide materials (U.S. Pat. Nos. 5,491,054 and 5,484,696), metal complexes (DE 3605279) or combinations of these including antioxidants (EP-A-0 203 746). All of these, even when including the use of high levels of solvent to help de-aggregate the dye, contribute to very small improvements in dye stability, with often little or no improvement in colour quality.
Another way of improving dye stability is to utilise UV absorbers, usually in a separate UV filtration layer, although it is sometimes beneficial to combine the UV absorbers in the same layer as the coupler.
One group of UV absorbers usually used in a separate layer are the 2-hydroxyphenyltriazines as described in DE 2113833 and DE 2230304. 2-hydroxyphenyltriazine materials in conjunction with magenta or yellow couplers to improve dye stability and Dmin yellowing have been disclosed in DE 4444258 and U.S. Pat. No. 5,541,045 respectively. DE 19538950 and DE 19701869 describe their use with phenolic and heterocyclic cyan couplers (although not pyrazolotriazole couplers, whose dominant mechanism of light-fade can be shown to be quite different), for dye stability improvements. DE 19701719 teaches that distinct dispersions of stabilizer and coupler in the same layer can provide improved dark storage and light stability of the cyan image but additional dye hue improvements via de-aggregation of the dye are never mentioned.
U.S. Pat. No. 6,242,169 discloses a colour photographic material containing a pyrazoloazole cyan coupler and a phenolic solvent, substituted in the para position with a nitrogen- or sulfur-bound group, the material having improved light stability. Although a phenoxy stabilizer may optionally be present, preferably in the same layer, there is no working example of its inclusion, no teaching that it would be in the same dispersion as the coupler, nor that hydroxy substitution would be favoured over, for example, alkoxy substitution.